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Why does the percentage of water in the body vary with body composition?

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The percentage of water in the human body varies primarily based on body composition because different tissues have significantly different water contents. Specifically, lean tissue is composed of approximately 75 percent water, whereas fatty tissue contains only about 20 percent water.1

Because of this difference, the total amount of body fat an individual carries directly determines their overall percentage of body water. Individuals with a higher proportion of lean body mass—such as many men compared to women—typically possess a higher percentage of total body water. Conversely, as body fat increases, the relative percentage of water in the body decreases. These proportions are dynamic and can be influenced by factors such as age, physical activity, and overall health, which affect the balance between lean tissue and stored fat.123